Influence of Issued Jet Conditions on Mixing of Confined Flows (original) (raw)
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Large-eddy simulation and laser diagnostic measurements of mixing in a coaxial jet mixer
Chemical Engineering Science, 2006
Numerical and experimental investigation of the turbulent mixing in a coaxial jet mixer is presented. Laser doppler velocimetry (LDV) and planar laser induced fluorescence (PLIF) were applied for measurements of velocity and scalar fields and their fluctuations. Numerical simulations were performed using large-eddy simulation and RANS with different closure models. These results are used for validation of numerical models and a detailed study of flow physics within the recirculation zone. ᭧
2014
Simultaneous velocity and concentration measurements of the isothermal mixing of a confined coaxial jet in air are performed in a small scale windtunnel at ambient conditions. The Reynolds number is 26000 based on the velocity difference of streams and on the diameter of the injection port. Instantaneous concentration measurements are imple- mented using planar laser induced fluorescence (PLIF) of acetone seeded injected flow. Simultaneous three-component velocity measurements are performed using stereo particle image velocimetry (PIV). The contribution provides details of the PLIF-PIV measurement setup and of the methodology to obtain instantaneous measurements of the local mixing fraction and velocity. Ensemble averaged velocities, concentrations, Reynolds stresses and turbulent mass fluxes are de- termined at four positions downstream of the nozzle using up to 3600 PLIF-PIV recordings. The convergence of the root-mean-square of velocity and concentration measurements is assessed ...
Mixing of confined coaxial flows
International Journal of Heat and Mass Transfer, 2006
The paper presents experimental results on the mixing process in a coaxial jet mixer in two mixing regimes. In the first mixing regime, a recirculation zone develops just behind a nozzle near mixer walls, while in the second regime a jet is mixed with the co-flow without developing a recirculation zone. In the both regimes, the mixing process is studied at Re d = 10 000. Behind the nozzle over the range 0.1 < x/D < 9.1, a velocity field in mixer cross-sections is measured by a one-component laser Doppler velocity meter and a scalar field is detected by the laser image fluorescence (LIF) method. A transverse autocorrelation function, integral length scales and probability density functions (PDF) are calculated using instantaneous distributions of a scalar and its fluctuations. It is shown that the scalar field acquires a homogeneous state faster than the velocity one. A quasi-uniform scalar distribution over the mixer cross-section is completed at the distance x/D = 5.1 in the first mixing regime, while this distribution has not been yet attained in the second. Analysis of the turbulent statistical moments and the autocorrelation function reveals how unsteady vortex structures exert a dramatic influence on the mixing. When the recirculation zone has developed, long-period antiphase oscillations exist near the mixer walls.
Analysis of the process of mixing of a passive impurity in a jet mixer
Journal of Engineering Physics and Thermophysics, 2007
The experimental results for two regimes of mixing of a passive impurity in an axisymmetric jet mixer-the mixing of a turbulent jet and a cocurrent flow to form a recirculation zone behind the nozzle and an analogous mixing without the formation of a recirculation zone (Re d = 10,000)-have been presented. The velocity field has been measured in the mixer cross sections at different distances from the nozzle (0.1 < x ⁄ D < 9.1) with a one-component Doppler laser anemometer, whereas the scalar field (concentration of the passive impurity) has been diagnosed by the laser-induced fluorescence method. Based on the scalar distributions obtained, the autocorrelation function and the integral scale have been computed, the form of the probability density function has been restored, and the distributions of the asymmetry and excess coefficients have been constructed. Visualization of flow in the mixer has been carried out. Introduction. Axisymmetric jet mixers currently used in the chemical and food industries are quite simple technical devices; they represent two pipes of different diameter installed coaxially. Despite their simplicity, they make it possible to implement both the laminar and turbulent mixing of media and to combine these processes by selecting the ratio of the flow rates of the initial media. In this work, we consider the mixing of a turbulent jet blown out of the nozzle with a cocurrent flow where the transient hydrodynamic regime of flow is implemented. The jet velocity exceeds the cocurrent-flow velocity. Possible mixing regimes are reduced to two fundamentally different types: (1) a recirculation zone of flow is formed immediately behind the nozzle and (2) no recirculation zone behind the nozzle is formed. Conditions for generation of these mixing regimes are determined from the flow-rate-to-diameter ratio [1]: (
Chemical Engineering Journal
Scalar mixing in a confined rectangular jet in crossflow
Journal of Fluid Mechanics, 2005
An experimental investigation of a confined rectangular jet in crossflow was performed. The rectangular jet is highly confined in that it spans almost 80% of the crossflow duct, rather than issuing into a semi-infinite crossflow. Furthermore, the jet is confined in the cross-stream direction because it issues into a relatively narrow duct. In addition, the flow rate of the secondary jet is large (up to 50% of the crossflow flow rate) which also influences the jet-crossflow interaction. Configurations of this type are found in a variety of different industrial manufacturing processes used to mix product streams.
Development of a turbulent jet generated by a mixer in weak co-flow and counter-flow
International Journal of Heat and Fluid Flow, 2000
Laser doppler velocity measurements of the axial, tangential, and radial velocity components were performed in a turbulent jet generated by an impeller operating in a weak co-¯ow and counter-¯ow. The measurements were carried out downstream of a model impeller placed in a glass-walled¯ume, which was either closed-o or operated with a small base¯ow. For the closed-o¯ume case a return¯ow was produced causing the jet to develop in a weak counter-¯ow, whereas in the base¯ow case the jet evolved in a weak co-¯ow. The jet development in these two types of¯ow, essentially representing perturbations of the classical problem of a jet in an uncon®ned, quiescent ambient¯uid, was compared with respect to mean velocities and integral¯ow properties such as jet spread, volume¯ux, and momentum¯ux.
Variable Viscosity Jets: Entrainment and Mixing Process
Whither Turbulence and Big Data in the 21st Century?, 2016
Turbulent jets have received considerable attention during the last decades. However, to our knowledge, one configuration has not received much consideration. The latter concerns the variableviscosity jet, wherein a turbulent jet of lower viscosity issues into a density-matched host fluid of higher viscosity. In this study, we carry out a comparison between Constant Viscosity Flows (CVF) and Variable Viscosity Flows (VVF), in a round jet, on the basis of the same initial jet momentum and the same initial Reynolds number. A propane jet issues into a N 2 (slight) coflow, for which the kinematic viscosity ratio is R v ≡ ν N 2 /ν propane = 3.5. The Reynolds number of the jet (based on the diameter, the initial velocity and the propane viscosity), is of 8000. The direct interactions between the velocity and the scalar fields reflect the need to perform simultaneous measurements of these two physical quantities. The stereo Particle Image Velocimetry (stereo-PIV) and the Planar Induced Fluorescence (PLIF) have been chosen for the velocity and the concentration measurements respectively. These diagnostics are detailed and the use of an original tracer for the PLIF measurements is notably brought forward. Experimental results are discussed, for both velocity and scalar fields, in the axial plane of the turbulent axisymmetric jet. It is shown that the presence of a strong viscosity discontinuity across the jet edge results in an increase of the scalar spread rate and of the turbulent fluctuations.
Effect of the separation wall in the mixing properties of coaxial jets
2012
The velocity and mixing field of two coaxial jet configurations has been experimentally characterized by means of hot-and cold-wire anemometry to investigate the effect of the initial conditions on the flow development, and to determine the leading processes in the "mixing transition" of the two streams. Amongst the possible operating conditions, four configurations with different inner and outer jet bulk velocity pairs (U i , U o) have been spatially characterized in terms of velocity and mixing field covering a region where 0.5 ≤ r u = U o /U i ≤ 3 with two different separation wall geometries, namely a sharp and a thick separating wall. For high velocity ratios the difference between the two geometries appears small and inside the experimental accuracy. On the other hand, for nearly unitary velocity ratio and with the thick wall configuration, the presence of a strong wake instability increases the interpenetration between the two streams enhancing the mixing process.
Experiments in Fluids, 2019
Simultaneous measurements of velocity and concentration using stereoscopic particle image velocimetry (stereo-PIV) and planar laser-induced fluorescence (PLIF) were used to investigate the mixing performance of a scaled-up multi-inlet vortex reactor (MIVR). Data were collected in three measurement planes located at different heights from the reactor bottom (¼, ½, and ¾ of the reactor height) for Reynolds numbers of 3250 and 8125 based on the reactor inlet velocity and hydraulic diameter. The collected data were analyzed to determine turbulent flow statistics such as turbulent viscosity, turbulent diffusivity, and turbulent Schmidt number. When analyzed across 16 different azimuth angles and radial positions (r) normalized by the reactor radius (Ro), the turbulent viscosity was found to be nearly axisymmetric. In the free-vortex region (r/ Ro > 0.2), the turbulent viscosity results were nearly constant. Near the center of the reactor in the forced-vortex region (r/ Ro < 0.1), the turbulent viscosity significantly increased, with peak values occurring near the center. The turbulent viscosity and Reynolds shear stress were highest near the reactor exit at the ¾ plane. The dominance of high turbulent fluxes and low concentration gradients near the reactor center led to high turbulent diffusivity. Away from the center, the turbulent diffusivity was reduced because of large concentration gradients and low turbulence intensity in the spiral arm region. The turbulent Schmidt numbers were also found to correlate with concentration gradients. The turbulent Schmidt number values were found to vary from 0.1 to 1.2. The highest spatial variation in Sc t was observed in the spiral arms region, where the concentration gradients are also the highest. This spatial variation in Schmidt number contrasts with the common assumption of constant Sc t in Reynolds-averaged CFD models.